Demonstration
Types of Depth Cues
| Researchers frequently categorize depth cues as binocular (dependent upon the use of both eyes) and monocular (available to either eye alone). Although we will describe all the cues listed, we will focus on the cues most helpful to the depth perception needed for sports. | |
Kinesthetic Cues from the Eye Muscles
Two of the depth cues are called kinesthetic cues because they depend on signals sent from the eye muscles to the brain.
Convergence, a binocular cue, is the degree to which the eyeballs must turn toward the nose (converge) in order to focus on an object. The closer an object is to you, the greater the convergence. If the object is more than 20 feet away, however, the degree of convergence is zero, so this cue is generally of little use on a sports field.
Accommodation is a monocular cue derived from the degree to which the lens needs to change in order to focus on an object. The closer an object is to you, the greater the accommodation of the lens. Because it's not a reliable cue, you probably don't use it much when other cues are available.
Retinal Disparity
Humans have two eyes located about 7 centimeters (3 inches) apart. This provides us with an extremely powerful binocular depth cue called retinal disparity. This cue is based on the disparity, or difference, between what the left eye sees and what the right eye sees. Each eye sees the same scene but from a slightly different point of view. Specialized neurons in the brain's visual cortex process this difference to construct a three-dimensional (3-D) view of the scene.
When two objects are the same distance from the viewer (as shown on the left below), the distance between retinal images in the right eye is the same as that of the left eye. When two objects are different distances from the viewer (as shown on the right), the distance between retinal images in the right eye is different from that of the left eye.
| Stereopsis is the perception of depth that is built up from retinal disparity, the difference in what the two eyes see. It is demonstrated in stereograms such as the Magic Eye illustrations that became so popular in the 1990s (see www.magiceye.com for some examples). Stereopsis gives us precise information about the relative depth of two objects, but it does not tell us much about the absolute distance between our eyes and the objects. |  |
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Two views of the same scene. The first shows what we would see if we closed our right eye;the second, what we would see if we closed our left eye |
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